Aspartate receptor is a dimeric receptor that binds aspartate with negative cooperativity. The structures of the periplasmic domain of aspartate receptor with zero, one and two aspartates bound per dimer have been solved by x-ray crystallography. Examination of the crystal structures shows that the two binding sites for aspartate are initially identical. Upon the binding of the first aspartate, there is a change in conformation that makes the empty site more crowded and hence less easy to the binding of the second aspartate. It has been demonstrated that Ser68 is integral to the allosteric switching mechanism in the aspartate receptor. Comparing the structures of the wild-type receptor and mutations at the 68 residue would shed light on the general mechanism of allosteric interactions.